This invention relates generally to Magnetic Resonance Imaging (MRI), and more particularly the invention relates to real time reduction of motion artifacts in MRI.
One of the most challenging areas in Magnetic Resonance Imaging (MRI) research is high-resolution imaging of the coronary arteries. The complexity of the motion of the heart and its surrounding structures due to cardiac and respiratory motion makes obtaining useful coronary artery images very difficult. If this motion can be temporarily paused, better images can be obtained. In essence, this is achieved through cardiac-gated, breath-held scans. Cardiac gating allows the acquisition of data frames at the same point in the cardiac cycle, thus freezing the cardiac motion with respect to the other data frames, while breath holding is an easy way to remove respiratory motion artifacts. Fast scanning techniques can produce useful images in the span of one breath-hold. In three-dimensional (3D) imaging and submillimeter resolution imaging, multiple breath-holds are usually required. In reality, the practice will be to use breath-holds when possible because they are easy and effective, however, movement that can occur at the latter part of a breath-hold and, in a smaller percentage of patients, an inability to maintain a long breath-hold or reproduce duplicate breath-holds can cause problems and require other methods to acquire useful coronary artery images.
Disclosed in U.S. Pat. No. 5,427,101 for xe2x80x9cDiminishing Variance Process for Real Time Reduction of Motion Artifacts in MRIxe2x80x9d of Sachs, Meyer, and Nishimura, is a method whereby motion can be detected in real time during the acquisition of MRI data. This enables the implementation of several algorithms to reduce or eliminate this motion from an image as it is being acquired. The method is an extension of the acceptance/rejection method algorithm called the diminishing variance algorithm (DVA). With this method, a complete set of preliminary data is acquired along with information about the relative motion position of each frame of data. After all the preliminary data is acquired, the position information is used to determine which lines are most corrupted by motion. Frames of data are then reacquired, starting with the most corrupted frame. The position information is continually updated in an iterative process, therefore each subsequent reacquisition is always done on the worst frame of data.
FIG. 1 is a flow diagram of the diminishing variance process disclosed in U.S. Pat. No. 5,427,101. Initially, a complete set of data frames, as required to construct an image, is acquired with each frame having a mechanism for indicating a relative position of the frame. A histogram of positional shifts in the frames is then developed, and the positionally worst data frames due to motion are then reacquired. The histogram is updated, and the positionally worst data frames in the updated histogram are reacquired. This process is continued until an image which is motion free is realized. As used herein, a data frame is a portion of the data, typically a single line in k-space for 2DFT imaging or a simple spiral interleaf for interleaved spiral imagery.
The present invention is an improvement to the diminishing variance process disclosed in U.S. Pat. No. 5,427,101.
In accordance with the invention, a diminishing variance algorithm (DVA) is used to operate on MRI data frames. A cumulative histogram is obtained from all data frames, and an image histogram is obtained using a limited number (e.g. 36) frames with the outlier or worst frames in the image histogram being continually replaced.
A target position is calculated for the imaged object based on the cumulative histogram. In accordance with a feature of the invention, in calculating the mode or target position the cumulative histogram is low-pass filtered to smooth the cumulative data. This can be done by averaging adjacent bin values. In accordance with another feature of the invention, the target position is recalculated based on the cumulative histogram throughout the scan, thereby allowing faster response to a patient""s movement and faster convergence based on the new target position.
The invention and objects and features thereof will be more readily apparent from the following detailed description and dependent claims when taken with the drawings.